How about WCF?

It's always great to see some .net action happening around ExtJS!

Though I think using async handlers is a bit dated approach. Did you think of writing WCF service for that purpose? It could help a lot in writing server-side code in more focused, service oriented manner.
It also has JSON support is built in and provides extended possibilities for easy exception handling.

Not really, I'm a bit out of the loop as far as .NET is concerned. Could you point me to some relevant examples? FYI the built in JSON support in .NET I found was reasonably poor, which is why I ended up using JSON.NET.

If you had a look at my Implementation of a Direct Router you may have seen that its really easy to let JSON.Net try to deserialize the json data to fit into the defined method parameters. This may shows a small part of the power of JSON.NET

Feature Request

To Start with, I am more of a VB user, so please forgive the fact that this is not in C#. As a feature request, why not create a DirectHandler as an IHTTPHandler that can do all of the work for the user, while grouping related function into a single file (and not having to expose every method in an Application to all pages)

Examples as follows.....

The DirectHandler to be added to the DLL (notice that I wrapped the DirectHandler and ApiHandler into one file, and check for the existance of any data. If none found, we assume you are looking for the API Definition).